1. Field of the Invention
This invention relates to an apparatus and process for drying cellulosic substances, such as pulp, paper sheet, and molded paper products, and textiles. More particularly, this invention relates to an apparatus and method of novel design that uses unsaturated water vapor (superheated steam) as the means to liberate both surface and bulk moisture from processed cellulosic and textiles materials in a continuous process. Even more particularly, this invention relates to a tunnel dryer designed to use steam as its drying means and assembling an array of novel features leading to greatly improved energy efficiency.
2. Description of the Prior Art
As a result of their fabrication process, all cellulosic products, be they pulp, paper web, or egg cartons, and textile products, have at some point a very high moisture content needing removal. This moisture-which typically constitutes 75% of the wet product weight-includes: (1) moisture on the surface of the product (surface water); and (2) moisture that is locked into the fibers of the product (bound water). The traditional means of removing this water has been to evaporate it by conveying the product through a tunnel dryer in which a hot gaseous drying medium is directed against the surfaces of the product, and, for some permeable materials, through the product. This drying by evaporation is extremely energy-intensive. The paper and pulp industry alone, with its large production volume consumes vast amounts of energy each year just in the drying stage of its production. Consequently, the reward for improving the drying efficiency is potentially very great. The applicant believes that the improved efficiency that his invention permits-particularly in the processing of molded paper products-represents a significant advance over conventional dryers, which consume about 120 Therms for every dry ton of product.
Traditionally, the drying medium of choice for tunnel dryers has been hot, dry air. The hot air technology developed decades ago during a period when energy costs were low compared to costs of construction and reflects this fact; it however continues to be used nearly universally in spite of its increasingly serious disadvantages. Among these disadvantages is the high cost of providing the energy to heat air as the drying medium, which, besides having a relatively low transfer efficiency, must be continuously replenished-as it is exhausted from the dryer with the water vapor that it has picked up from the items to be dried. (Once the air has become laden with water vapor as well as reduced in temperature, its drying capacity drops precipitously.) In addition, air as a drying medium has the potential to over-dry the products, thus embrittling them and/or imposing internal stresses that reduce their value. What is needed is a drying medium that has a heat transfer efficiency higher than air and that can be recycled. The closed system that the recycling requires leads in turn to the necessity of dealing with the water vapor removed from the products and entrained in the drying stream-water vapor that traditionally has just been vented into the atmosphere outside the dryer, with the energy required to evaporate it. Also needed is a means of allowing easy tunnel ingress and egress to the conveyor belt while keeping air out of the tunnel and the hot drying medium in.
The present invention uses gaseous H.sub.2 O at a pressure of one atmosphere and at temperatures and densities that ensure that the gas is extremely undersaturated. (Stated differently, the drying medium is gaseous H.sub.2 O at temperatures far above the dew point. Such atmospheres-be they at 70.degree. F. or 700.degree. F.-are referred to as superheated steam.) The invention consists of the drying process--designed to achieve a very high energy efficiency--and the apparatus needed to implement the process. How this is done can best be seen after a more detailed examination of what is entailed in drying cellulosic (and other fibrous) products.
When the wet product is introduced into the tunnel, it has water standing on its surface. The first stage of the drying consists of the evaporation of that surface moisture. During this first stage, the rate of water removal remains constant, and at a level that is a function of the mass per unit time of drying medium impinging on the product. Surface moisture removal continues at this constant rate until the surface moisture is gone, by which point the product's total moisture content is reduced to approximately 30% of the total weight of the product (the exact percentage at this stage depending on the particular item involved). The moisture remaining is in the bulk of the product, contained in its fibers; the removal of the bulk moisture depends upon capillary action to draw it along the fibers up to the surface, where it is vaporized by the drying medium. As a rule, the rate of water removal falls precipitously and continues to fall as the bulk moisture is being removed. The amount of energy required to liberate this bulk moisture is a function of the length of the fibers of the product and other factors affecting the "wick efficiency." In the dryers currently used in the pulp and paper industry-those using air as the drying medium-the energy required to remove the bulk moisture is approximately equal to that required to remove the surface moisture, in spite of the latter comprising a much larger quantity. Unlike the case of the surface moisture, the rate of removal of bulk moisture is not directly proportional to the rate at which the drying medium impinges upon the product.
When air is used as the drying medium, the product (consisting either of discrete items such as molded paper/pulp products or of a continuous sheet such as paper web, raw pulp or textiles) is conveyed down the tunnel with a stream of hot, dry air impinging on it. Because of the once-through path for the air, which comes in dry and exits carrying away vaporized moisture, the product is exposed at each step of the way to very low humidity air. Products thus dried often exit the drying chamber with their moisture content reduced to approximately 1-3% by weight. Once out of the chamber and exposed to ambient air, they regain moisture up to some equilibrium point (about 6-8% moisture by weight), though not instantaneously and not uniformly. In this way, air as the drying medium in current systems may over-dry the product to the point where natural atmospheric conditions replace moisture that the dryer has had to expend significant energy to remove. If this over-drying occurs, moisture is reintroduced by the ambient atmosphere in an uncontrolled fashion that can set up internal stresses leading to product warpage and other deleterious effects rendering the product less than satisfactory.
While this problem can be controlled in present hot-air dryers by regulating the dwell time of the product within the dryer, a further problem exists with such dryers. In particular, high-speed, hot-air drying tends to case-harden and warp the surface of the product. This leads not only to poor product quality, it may also, in effect, entrap moisture by reducing the wicking efficiency of the product fibers. As a result, it becomes more difficult for bound water to escape to the surface to be vaporized. (Entrapping bound moisture in this manner leads to even poorer quality products.)
It is well-known that steam (that is, gaseous H.sub.2 O-not to be confused with the airborne liquid water droplets known vernacularly by the same name) transfers heat more efficiently than does air. Steam at a temperature T will exchange more heat with a surface it is in contact with than will air at the same temperature, all other things being equal. That is not the whole story, of course, since it takes more energy to heat up a unit volume of steam to temperature T than it does to heat up the same volume of air to the same temperature; also, since the ultimate goal is to dry the product, the degree of saturation of the steam atmosphere is a very important parameter to control when steam is the drying medium. From thermodynamic considerations it is seen that the heat transfer efficiency depends upon the enthalpy of the fluid (air or steam). At a pressure of one atmosphere and temperatures above 375.degree. F., steam has an enthalpy at least 30% higher than that of hot air. If the gas is a mixture of air and steam, the enthalpy is intermediate between that for steam and that for air. Consequently, another consideration that must be dealt with in designing a dryer using steam as the drying medium is maintaining the purity of the steam; the extra capital expense involved in such systems can result in a net loss in efficiency if the steam becomes significantly contaminated with air. Since it is essential in such systems that the drying medium be continuously recirculated, the maintenance of the steam's air-free status is a serious problem; a small air leak can over a period of time significantly dilute the recirculating steam. (This is a difficulty that does not arise in systems where the drying medium just goes once-through and then out the other end of the dryer.)
As a theoretical idea, the use of steam as a drying medium is not new. This is true even within the field of pulp and paper drying (though there is apparently no prior art addressed at drying molded paper products and the special problems that this entails). See, for example, Dungler I (U.S. Pat. No. 2,590,849--issued Apr. 1, 1952), which teaches a method for continuous drying using steam as the medium. Although Dungler I is concerned almost exclusively with textiles it also alludes to paper and other fibrous materials-but only those that can be drawn continuously through the drying tunnel and are sufficiently permeable that the drying medium can impinge them at high velocity. In particular, Dungler I suggests using steam to dry thin sheets of material or paper, whereby the item to be dried is affixed to a permeable conveyor belt and superheated steam blown through it to liberate bulk moisture. U.S. Pat. No. 2,682,116 issued to Dungler in 1954 (Dungler II) discloses apparatus for effecting the method disclosed in Dungler I. Both the apparatus and method claimed in Dungler II relate to very high impingement velocities and to the establishment, using a complex vacuum generating system, of a pressure differential across the web of product to be dried. The apparatus and method of Dungler II would be completely inapplicable to the drying of molded paper products, which are not continuous and which are impermeable even to the high impingement velocities envisioned by Dungler II. In addition, those high velocities would compromise the position integrity of such products as they are conveyed through the dryer. A similar approach is used by Gillis (U.S. Pat. No. 2,760,410), which discloses particular plumbing and vacuum arrangements for the use of 400.degree.-1500.degree. F. steam to dry continuous webs of pervious paper. Luthi (U.S. Pat. No. 4,242,808, 1981), claims a method and apparatus for using steam to dry paper web, either pervious or impervious to the drying medium. Luthi recognized the efficiency associated with superheated steam heating, and goes further than Dungler I and Dungler II in noting that the steam must not be contaminated with air if the drying medium is to be as efficient as possible; nevertheless, Luthi does not set out the particular techniques that are necessary to ensure minimum air contamination. Also, Luthi with its high impingement velocities is inappropriate to the drying of individual items.
Although the process of drying certain pulp and paper products with steam has been disclosed in principle in the above-cited prior art, it has rarely if ever been reduced to practice commerically. Within the molded paper product industry the process has never been developed even in theory, and it is the molded paper product industry that will be burgeoning during the coming years and in need of industrial processes that are far more energy-conserving than those used in the past. That is, current environmental concerns associated with plastic containers has led to the reintroduction by large-scale users of food containers-especially the fast food outlets-of molded cellulosic articles, bringing pulp and paper processing plants under increasing pressure to develop their capacity to handle such products efficiently. More efficient drying will play a key role in overall efficiency, and it is submitted that more efficient drying will use steam as a drying medium. However-and as alluded to above-because molded products are generally much thicker than paper web and sheet, use of the prototypical steam dryers taught by the prior art cited above would require even greater steam velocities or longer dwell times to drive out trapped moisture. This would increase the likelihood that the molded articles would be displaced on the conveyor or blown off completely. A further problem associated with increased steam output is the greater rate of energy production needed; this in turn puts a greater premium on steam recovery techniques, something that has not traditionally been a significant concern. To realize the goal of greater overall energy efficiency through the use of steam in connection with the drying process, the energy used to produce the superheated steam must be recovered from the sheet or article after the drying has taken place. This leads to still another problem-the need to provide structurally sound piping systems and sophisticated steam recovery devices-items that drive up the cost of a steam dryer and drive down its desirability in any trade-off analysis. Therefore, while a steam dryer is in theory more efficient than a hot-air dryer, an evaluation of the two methods must consider: (1) the possibility that two types of steam dryers would be required-one for continuous sheet or web and one for molded articles; and (2) that the hardware required to dry with superheated steam would be much more expensive.
It is plausible that the failure to implement steam drying even for pervious and continuous paper webbing is attributable to the costly dryer sizes and structures required by steam systems. A truly efficient steam dryer requires a chamber and an associated air-channelling system, both of which are essentially air-tight and which are capable of withstanding temperatures above 375.degree. F., and up to 1600.degree. F. Another problem-and one associated with pulp and molded articles in particular-involves the increased difficulty in drying products that are much thicker than simple fabrics and paper webs. Increased thickness of the item to be dried means that much higher impingement velocities--or longer dwell times--are needed to establish and maintain the heat transfer rate needed to extract the moisture trapped within the product. Higher velocities tend to blow molded articles off the conveyor that carries them along in the continuous drying process. Even slight shifts in the position of the articles are serious since in the continuous processing of such discrete articles it is extremely important to maintain position integrity; automatic stackers pick the articles off the conveyor as they come out of the dryer. These types of products present still another problem to continuous drying, either by steam or by air-they vary in size and shape to such an extent that variations in drying times generally require varying either the length of the dryer or the dwell-time within the dryer. Therefore, while the superheated steam drying discussed by Dungler and Luthi is possible in theory, there are practical problems associated with the use of steam that must be addressed before a practical reduction to practice can be achieved. Furthermore, there are theoretical problems associated with the use of steam to dry individual molded items that must be addressed.
What is therefore needed is an industrial dryer for paper and pulp products that is more efficient than conventional dryers, especially where molded paper products are involved. In particular, what is needed is an industrial dryer using superheated steam as a drying medium and that can be effectively integrated into the complete manufacturing process to the extent that heat and water generated by the dryer is used in other parts of that process. More particularly, what is needed is such an industrial dryer which can--with a minimum of disruption--replace the conventional hot air dryers currently installed within the textile and pulp and paper industries, so as to reduce the vast and growing energy expenditure which that industry demands.